Exploration for gas frequently involves significant well depths, coupled with hostile conditions such as high pressures and temperatures. Additionally, the gas may be "sour," further contributing to a hostile environment for seals. In order to produce from zones which may be as deep as 20,000 ft below the surface or more, where downhole temperatures can reach 500.degree. F. or more and differential pressures can be as high as 20,000 psi, designs employing chevron-type seals have been used to seal between a liner and the production tubing. Such assembly of chevron seals is illustrated in FIG. 1. The production tubing (not shown) is connected to a mandrel 10. A lower travel stop comprises rings 12 and 14 which are threaded together at thread 16 over key 18, which extends into a keyway 20 in mandrel 10. Above the assembly of rings 12 and 14 is an extrusion ring 22. Extrusion ring 22 can be made from 25% glass-filled teflon, used with or without a metal back-up ring, or alternatively, a material known as PEEK (polyether-ether-ketone). Its purpose is to prevent extrusion of rings 24. In the past, the service temperature and differential pressure determined some of the materials used for the seal illustrated in FIG. 1. For example, for services of about 450.degree. F. with differential pressures of about 15,000 psi, extrusion ring 22 was made from PEEK. Above extrusion ring 22, a stack of upwardly oriented chevron packing rings 24, made preferably from a composite material known as molyglass, which is a composite of teflon, fiberglass and molybdenum disulfate, was used. This material provided excellent chemical resistance to sour gas formation fluids, acids as well as other treating fluids, in combination with the necessary thermal and mechanical properties for a sealing system for the parameters stated. Above the stack of upwardly oriented chevron seal rings 24 was an O-ring 26, separating all the upwardly oriented chevron rings 24 from the downwardly oriented chevron rings 28. Above the downwardly oriented chevron rings 28 was an upper extrusion ring 30, followed by ring 32 engaged to ring 34 at thread 36 over key 38, which extended into a keyway 40 in mandrel 10. Extrusion rings 22 and 30 were interference-fit onto mandrel 10 and capable of some movement during the installation of the mandrel 10 into a liner bore (not shown).
In the past, in an effort to ensure that a sealing assembly, such as that shown in FIG. 1, would effectively seal between the mandrel 10 and the liner, multiple stacks of such seals 24 or 28 as shown in FIG. 1 were used. Sometimes as many as 20 different stacks would be attached to a mandrel 10 for interaction with the liner bore with the hope that adequate sealing bidirectionally would be obtained from at least one of the assemblies. With such adverse conditions, reliability of the seal assembly shown in FIG. 1 was of great concern, necessitating numerous back-up assemblies mounted to the same mandrel 10. The opposite orientations of chevron seals 24 and 28 were required for the purpose of sealing against differential pressures in either direction. The chevron seal stack 24 was useful in sealing against differentials involving higher uphole pressures, while the stack 28 was useful in sealing against differential pressures with higher downhole pressures.
Typically, the production tubing would be assembled at the wellbore and gradually lowered into position in the liner to seal off the production tubing against the liner at the desired depth. This initial assembly could result in the upper end of the production tubing being in the wrong position with respect to the rig floor. If this situation occurred, the assembly of seals as shown in FIG. 1 would have to be disengaged from the liner bore so that the proper end joint at the surface could be installed to get the appropriate terminal height for the production tubing with respect to the rig floor. The placement or "stabbing in" of the stacks of seals as shown in FIG. 1 in high-temperature environments proved to be detrimental to the reliability of such seal assemblies to seal effectively between the production tubing and the liner bore.
Several problems were encountered due primarily to the high-temperature environment, as well as various hydraulic phenomena which acted to defeat the proper placement of the downwardly oriented chevron rings 28 with respect to the liner bore.
When using repetitive stacks of seal assemblies such as that shown in FIG. 1, the lower-most seal assembly would obviously be the first to engage the liner bore, where its diameter is reduced for seal contact. The upwardly oriented chevron seals 24 would have to fit into a liner bore which, for the purposes of minimizing extrusion, was only slightly larger than the retaining ring 22. Each of the upwardly oriented chevron rings would flex as the mandrel 10 was advanced into the seal bore in the liner. Since each of the chevron rings 24 had a cutout 42 separating an internal wing 44 from an external wing 46, the external wings 46 would readily flex inwardly toward mandrel 10 as mandrel 10 was advanced into the sealing bore of the liner. The chevron stack 24 could also shift upwardly in response to downward movements of mandrel 10. The extrusion ring 22 could also move slightly upwardly in response to the same downward movement of mandrel 10, trying to seat off the upwardly oriented chevron rings 24. Upon further advancement of mandrel 10, the lower-most downwardly oriented chevron ring 28 would have to have its outer wing 48 compressed so that it could fit into the liner seal bore. However, at that point in time, the liner bore would be filled with well fluids located adjacent O-ring 26. Experience has shown that in certain applications, further advancement of the mandrel 10 resulted in a build-up of hydraulic pressure adjacent O-ring 26, which had the disadvantageous effect of forcing outer wing 48 on not only the first but the entire stack of downwardly oriented chevron rings 28 in a counter-clockwise direction. Accordingly, rather than being installed in the liner bore in the position illustrated in FIG. 1, all of the outer wings of the chevron rings 28 would instead be deflected so that they would contact the liner bore in an upwardly oriented position, in essence bent back counter-clockwise to fit into the liner bore. Once inserted into the liner in this rotated position, the ability of rings 28 to seal against differential pressure coming from downhole was essentially defeated. The reason that this occurred was that each individual chevron ring 28 could not overcome the hydraulic pressures generated in trying to displace the liquid volume below the chevron rings 28, which occurred while trying to advance those very same chevron rings 28 into the liner bore for sealing. The component nature of the stack of chevron rings did not provide sufficient individual rigidity in each ring 28 to allow the outer wings 48 to overcome hydraulic forces present in the liner bore to prevent the adverse counter-clockwise deflection. This situation was further aggravated with similar stacks of seals such as those illustrated in FIG. 1 but located further up on mandrel 10. Clearly, once the first seal assembly as shown in FIG. 1 would seat against a liner bore, further advancement of mandrel 10 would clearly not allow any well fluid to be displaced downwardly beyond the first seal assembly which had already seated against the liner bore. What was needed and found lacking in the prior design was sufficient structural rigidity for the downwardly oriented chevron seal members 28 so that they could withstand the hydraulic forces placed on them as the mandrel 10 was being advanced into the liner bore for sealing therewith.
The seal element of the present invention addresses the issue of the required rigidity so that the sealing element properly goes in its desired location between the mandrel 10 and the liner bore and effectively enters the liner bore, retaining its initial shape so that effective sealing against differential pressures in either direction can be accomplished. By increasing the reliability of the seal between the liner and the mandrel, significant expense reductions can be recognized by reducing or perhaps eliminating back-up sealing assemblies on the mandrel 10. In another feature of the invention, low-pressure pockets are created between the seal member and the mandrel, thus inducing built-in pressure differentials which tend to use the energy of the surrounding well fluid to act against the seal member to reduce its profile. This facilitates insertion of the seal into a liner bore, which frequently involves very close clearances in order to effectively address the concern of potential extrusion. Various embodiments of the invention are disclosed, some of which are unidirectional and are used in opposed stacks, while others are bidirectional and comprise of a single sealing element. An additional interference back-up sealing feature is provided with each seal member to further assist in sealing against the liner bore.